AMMAN (Reuters) - A prominent leader in Syrian al Qaeda offshoot Nusra Front was killed on Sunday in what rebel sources said appeared to be a U.S. drone strike in the rebel-held northwestern province of Idlib.

The Syrian Observatory for Human Rights, which tracks violence in the country, confirmed reports on websites by militant sympathizers that Abu Firas, “the Syrian”, was killed along with a number of his companions.

While the Observatory said he was killed in a suspected Syrian or Russian air raid on a village northwest of the city of Idlib in northwestern Syria, two rebel sources said the attack appeared to have the hallmarks of a U.S. drone strike.

“The Sheikh was with his son and several companions,” said one of the rebel sources familiar with details of the incident.

A U.S security official said the United States was aware of reports about Abu Firas’ demise but had no information to offer on Sunday. Another source did not rule out it could be a Syrian strike.

Islamist rebel sources said Abu Firas was a founding member of the militant group who fought in Afghanistan in the 1980s and was a senior member of its policy-making Shura Council.

He worked with al Qaeda leader Osama bin Laden in galvanizing support among Pakistani supporters of the fundamentalist Taliban movement in Afghanistan several decades ago, the sources said.

They added that Abu Firas, who was a former Syrian army officer discharged in the late 1970s because of his Islamist leanings, played a significant role in training Muslim Sunni jihadists who came from many parts of the Arab world to Afghanistan to fight the Russian occupation of the country.

Abu Firas had many followers within the hardline group and gave commentaries released by Nusra Front on issues ranging from governance to religious jurisprudence, the rebel sources said.

Originally from Madaya, near Damascus, Abu Firas was a fervent opponent of Islamic State’s style and ideologically at odds with the militant group that occupies parts of Syria and Iraq.

“May God accept him as a martyr, he was a commanding figure. This was engineered by the Crusader axis,” said one of the sources.

A fragile “cessation of hostilities” has held in Syria for over a month as the various parties try to negotiate an end to Syria’s five-year-old civil war.

But the truce excludes Islamic State and Nusra Front, and air and land attacks by Syrian and allied forces continue in parts of Syria where the government says the groups are present.

Reporting by Suleiman Al-Khalidi, Additonal reporting by Mark Hosenball in Washington; Editing by David Evans and Peter Cooney